CD welders have been used in a variety of industries for many years. The traditional weld process was to charge a capacitor bank to a relatively high voltage (hundreds or even thousands of volts), then fully discharge the capacitor bank through the pulse transformer and into the part to be welded. For improved welding results, many controls offer dual pulse welding capabilities wherein this process is repeated twice for each part to be welded. The first pulse is typically done at a low energy to “condition” the parts to be welded. The second pulse is then done at a higher energy to complete the weld.
CD welders have the advantage of an output that is independent of the local line voltage. The charging circuit charges the capacitor bank up to a user-programmed voltage. The energy delivered by the welder depends only upon the value of the capacitors and their voltage by the formula E=½ CV2 where E is the energy, C is the capacitance, and V is the capacitor bank voltage. As the line voltage varies, the time to charge the capacitor bank may vary, but the overall energy delivered does not vary. This is an advantage when compared with AC type welders that draw their energy directly off the incoming line and are susceptible to output variation when the incoming voltage varies during the weld.
One alternate weld process is known as arc welding. A welding power supply is used in arc welding to generate an electric arc between an electrode and the base material to melt the metals at the welding point. The arc welding process requires an ongoing electric current (either direct (DC) or alternating (AC) current) to maintain the arc for a substantial period of time. This contrasts with capacitor discharge welders, which are single-shot devices. It should be noted that in arc welding, the parts to be joined are heated by the electric arc, and not by the resistance of the parts to be joined as the current passes through them. In addition, the voltage required to strike and maintain this arc is typically much higher that for capacitor discharge welders. Striking an arc can require 80V or more, while capacitor discharge welding is done at 10V or less. These differences mean that although similar electronic components may be used in both types of welders, the details of how they are used differ significantly.
Typical CD welders today use a type of Silicon Controlled Rectifier (SCR) to connect the capacitor bank to the pulse transformer. A characteristic of these switches is that they can be turned on with a control signal input, but that they cannot be turned off until the current through them goes to zero. In short, they are turned on, and stay on until the current reaches zero (as when the capacitor bank is nearly depleted).
Prior art also includes CD welders that used thyristors (Gate Turn Off or GTO devices) to connect the capacitor bank to the pulse transformer. However, these devices are typically used in the same manner as the SCR described above.